1. Field of the Invention
This invention relates to a display controller capable of driving either of a CRT display unit or a liquid crystal display unit.
2. Prior Art
A visual display unit is an indispensable terminal device for a computer system such as a personal computer and an office computer. Until recently, CRT (cathode-ray tube) display units were used in most of the computer systems as such terminal devices. However, the recent advance in the liquid crystal display technology have made it possible to use liquid crystal display units as the terminal devices of computer systems particularly of a portable type.
There has been proposed a display controller formed in an LSI device which can drive a display unit in accordance with commands fed from a central processing unit connected thereto. For example, a .mu.PD3301 manufactured by Nippon Electric Co., Ltd. and an HD46505 manufactured by Hitachi, Ltd. can control CRT display units, and a .mu.PD72030G manufactured by Nippon Electric Co., Ltd. and an HD61830 manufactured by Hitachi, Ltd. can control liquid crystal display units.
A commercially available liquid crystal display unit is generally composed of two (upper and lower) screens which are independently driven, while a CRT display unit can be driven as one screen. Thus, a CRT display unit and a liquid crystal display unit must be driven in different manners from each other. Also, the manner of reading data from the video memory for a CRT display unit is different from that for a liquid crystal display unit. For this reason, each of the aforesaid conventional display controllers is designed so as to drive only one of the two kinds of display units. As a result, when it is desired to change the display unit used to another display unit of a different type, not only the display controller but, also the control program therefor must be changed.